Antenna tuner units (ATU) are currently being considered for use in mobile terminals used for mobile radio communications. One purpose of an ATU is to match the impedance between a power amplifier (PA) and an antenna, thereby maximizing total radiated power (TRP). Another purpose of an ATU is to increase reception of radio signals by maximizing total isotropic sensitivity (TIS) for the antenna, especially since the antenna can experience large voltage standing wave ratio (VSWR) changes. For example, an antenna's input impedance is one parameter that can be affected by changes in a user's body placement versus the antenna. Certain body placements relative to the antenna will result in decreased radiated power due to a relatively large amount of power being reflected off the user's body, thus limiting the antenna's TRP. When the antenna is used for signal reception, other body placements relative to the antenna will reduce the TIS, resulting in poor receiver performance.
Presently, ATUs for cellular applications or mobile Internet devices (MIDs) that use third generation (3G) or fourth generation (4G) cellular systems are dual purpose ATUs. These ATUs are dual purpose in that they are used for impedance matching in both a transmit path and a receive path between a transceiver interface and a communicatively coupled antenna. These dual purpose ATUs require tuning elements that can handle large voltages due to the relatively large transmit powers involved. For example, dual purpose ATUs must be able to withstand a 6:1 VSWR. At a +33 dBm output power radiated from an antenna, a 6:1 VSWR requires a peak-to-peak voltage of 70Vpk-pk. Micro-electromechanical systems (MEMS) switches having a large voltage drive have been developed to handle such large voltages. A disadvantage of MEMS switches is cost. Solid state switches using silicon-on-sapphire (SOS) or silicon-on-insulator (SOI) can also be used to handle large peak-to-peak voltages. However, at least fourteen cascode switches per switch branch must be used in order to handle 70Vpk-pk. Thus, a die size for dual purpose ATUs must be relatively large, on the order of 3 mm2. Due to such a relatively large die size, SOI technology may be required, which further increases cost. In a further complication, dual purpose ATUs in some cases requires coupling and detector circuitries to calculate optimum tuning settings.
ATUs are presently being considered for MIDs in order to enhance the TRP/TIS of mobile terminals for the purpose of providing better high speed data operation. However, presently considered ATUs are designed to adjust impedance matching between an antenna and a radio interface to provide tuning for both a transmitter circuit and a receiver circuit. Thus, due to the large transmitter power, a relatively expensive dual purpose ATU solution is most often deemed necessary. However, a user usually operates a mobile terminal in a high speed data downlink mode. Moreover, with the advent of 4G mobile terminals like long term evolution time division duplex (LTE-TDD), which have higher modulation bandwidth and higher downlink data rates, the quality of downlink performance is critical. As a result, improvements in TIS are more critical than improvements in TRP. Therefore, what is needed is an ATU that includes a receive only tuner circuit for a mobile terminal that has increased TIS along with reduced cost of implementation.